Understanding the effect of magnetic field configuration on preferential transport of impurities in PISCES-RF*

Helicon plasma sources driven by the radio frequency of 13.56 MHz have proven to be an efficient way of producing high density plasma (n_e ≥ 10¹⁹ m^-3) in linear plasma devices^1–3. Such devices are being built to study the plasma-material interaction under fusion reactor relevant conditions. A helicon source when operated under high RF-power¹ was found to generate impurities originating from its vacuum-window (e.g. Al₂O₃, AlN, Si₃N₄) as a result of the strong rectified sheath voltage on the ceramic. Previous studies revealed that the helicon window is eroded by deuterium and oxygen¹. However, self-sputtering of Al or Si could contribute significantly. The present study reports on the transport of the released impurities towards the plasma upstream side in PISCES-RF device². Deposition of the Al-impurities were traced at various locations of the vacuum vessel. A strong deposition of Al was observed on the upstream side walls when the magnetic field is terminated at the sides wall in a “cusp” configuration. The physics of the impurity generation and its transport in PISCES-RF will be presented and corroborated with modeling of the RF sheath, plasma-, neutral and impurity transport for various magnetic field configurations.

References:

¹Beers, C. J. et al. Phys Plasmas 28, (2021).

²Thakur, S. C. et al. Plasma Sources Sci Technol 30, (2021).

³Caneses, J. F., Blackwell, B. D. & Piotrowicz, Phys Plasmas 24, (2017).